wiki:PetascopeTimeHandling

Version 10 (modified by Piero Campalani, 10 years ago) ( diff )

t-crs section

Table of Contents

  1. Temporal CRSs
  2. Using and understanding time in WCS/WCPS
  3. Irregular time series

Time dimension(s) handling in Petascope

In this page we will describe how time can be embedded in the definition of a spatio-temporal coverage for WCS and WCPS web services, by means of temporal Coordinate Reference Systems (CRSs).

Temporal CRSs

Thanks to the [SecoreUserGuide SECORE] resolver, time can be embedded in the n-dimensional CRS space just as another dimension: temporal CRS definitions can be used in composition with geodetic ones to create a unique space-time aquarium for the coverage.

Temporal CRSs (gml:TemporalCRS) are always 1D and are rather simple entities: in a few words, they count the amount of a certain time step from an epoch. No stratigraphic calendars, leap seconds, eras and eons are involved here: simple linear counting of time units. Time units are the Unit of Measure (UoM) of the reference system.

What is a temporal CRS.

Like geodetic datums bind the set of geographic coordinates to an absolute position on Earth (or planetary object), temporal datums link the temporal axis to an absolute moment in time. Indeed a temporal datum is essentially an "origin" (the epoch) expressed through an ​ISO:8601 datetime representation. Technically, this representation is of type ​xsd:dateTime, which says:

The type xsd:dateTime represents a specific date and time in the format CCYY-MM-DDThh:mm:ss.sss, which is a concatenation of the date and time forms, separated by a literal letter T. All of the same rules that apply to the date and time types are applicable to xsd:dateTime as well. An optional time zone expression may be added at the end of the value. The letter Z is used to indicate Coordinated Universal Time (UTC). All other time zones are represented by their difference from Coordinated Universal Time in the format +hh:mm, or -hh:mm. These values may range from -14:00 to 14:00. For example, US Eastern Standard Time, which is five hours behind UTC, is represented as -05:00. If no time zone value is present, it is considered unknown; it is not assumed to be UTC.

At the time being, ​we provide a first basic set of Temporal CRSs (T-CRSs) which (among others) are under review process by the OGC-NA authority and represents part of the effort of the newly born (public) OGC ​Temporal Domain Working Group. These definitions are:

  • ​ANSI date (OGC:AnsiDate) : linear count of days (labelled 'ansi') since 1st of January 1601, 00h00 UTC (note that decimal coordinates are possible to reference fraction of the day).
  • ​ANSI date (OGC:Temporal) : parametrized CRS for the user-friendly definition of a custom T-CRS: it resolves to a concrete definition of T-CRS as long as the epoch and the UoM are explicitly set in the query of the URI (and optionally an axis label and an axis direction: defaults are 't' and 'future'). UoMs can be defines as either ​UCUM c/s codes or actionable URIs pointing to valid GML definition of UoMs; UCUM codes or URIs like ​http://www.opengis.net/def/uom/UCUM/0/<code> (which the OGC SISSVoc redirects to the UCUM web page) or codes are suggested here: our ISO timestamp parser relies on UCUM codes too.

For a full list of time resources under review by OGC-NA please visit ​this page (active participation is also appreciated via ​mailing list). After official acknowledgment of such resources, all of them will be provided by our resolver and additionally by the official 'opengis' servers.

Still, especially thanks to our customizable OGC:Temporal CRS, it is possible to start deploying spatio-temporal (but also purely temporal) coverages via rasdaman/Petascope.

While all kinds of T-CRS are possible with OGC:Temporal, you might decide to create your own definitin so to have a more meaningful URI. This can be easily achieved: in spite of the verbose GML representation, what is actually important for Petascope in a T-CRS is:

  • the datum origin (we assume UTC default)
  • the axis time step (`gml:CoordinateSystemAxis/@uom)
  • the axis label (gml:axisAbbrev)

After defining it, then you might want to replace the equivalent Unix Time T-CRS with OGC:Temporal with your URI:

Parametrizations are also useful when one needs to compound together more than one T-CRS of the same kind: this can happen especially e.g. when numerical models are applied daily (real time axis) targeting a set of future days (simulation time axis). Using the same T-CRS makes sense here (let's pick OGC:AnsiDate for example), but in order to avoid a clash in the labels of the axes we need to actually use two different URIs in the composition of the CRS, for instance:

For further info on CRS parametrization and composition, see the SECORE [SecoreUserGuide user guide] and also IndexCrss section.

Using and understanding time in WCS/WCPS

..requests: double-quotes switch …support of time zones …ISO date formatter with optional time (taken from ​Joda-Time API): 

date-opt-time     = date-element ['T' [time-element] [offset]]
date-element      = std-date-element | ord-date-element | week-date-element
std-date-element  = yyyy ['-' MM ['-' dd]]
ord-date-element  = yyyy ['-' DDD]
week-date-element = xxxx '-W' ww ['-' e]
time-element      = HH [minute-element] | [fraction]
minute-element    = ':' mm [second-element] | [fraction]
second-element    = ':' ss [fraction]
fraction          = ('.' | ',') digit+

For examples see our WCS systemtest queries n: 27-28, 35-37.

..responses: numeric time coords encoded with origin + offset vectors (+ foreseen the use of gml:EnvelopeWithTimePeriod #678)

Irregular time series

..independent of time: irregular aligned grids can be defined (see Petascope [PetascopeUserGuide user guide].) the CRS is also independent of the topology of the referenced grid.. To import … additional coefficients will be … so that the numerical coordinate is: formula: ...

See also rasgeo [RasgeoUserGuide user guide].

See attachments for farther resources on the topic.

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